Relaxation techniques are widely used to determine the mass transport parameters of oxygen, namely the chemical surface exchange coefficient k(chem) and the chemical diffusion coefficient D-chem, of mixed ionic-electronic conductors. The investigation of technologically relevant mixed conducting oxides that exhibit high values of both k(chem) and D-chem by relaxation methods faces the problem that the amount of oxygen released or taken up by the sample during a relaxation experiment is not negligible. In fact, it can be of the same order as the amount supplied by the gas stream; the desired step-like change in oxygen activity that initiates the relaxation process thus becomes ill defined. In this study we examine strategies to identify and counteract this problem: As a model system we use the mixed ionic electronic conducting perovskite-type oxide Ba0.5Sr0.5Co0.8Fe0.2O3 (- delta), which exhibits extraordinarily high oxygen exchange kinetics, and we perform conductivity relaxation experiments that monitor the oxygen partial pressure in the vicinity of the sample. (C) 2015 Elsevier B.V. All rights reserved.